1. Field of the Invention
The present invention relates to a film-covered electric device in which casing films encapsulate therein an electric device element, such as a battery and a capacitor.
2. Description of the Related Art
Recently, there has been a growing demand for lighter weight and thinner dimension of a battery as a power source for portable equipment or the like. As a casing for the battery, a laminated film structure has been increasingly used instead of a conventional metallic can, because the laminated film structure is possible to achieve lighter weight and thinner dimension and have flexibility in forming shapes, compared with the metallic can, which has some limitations in achieving lighter weight and thinner dimension with less flexibility. The laminated film includes a metallic thin film layer or metallic foil and thermo-fusing resin film layers.
The laminated film structure used in the battery casing typically includes a thermo-fusing resin film as a heat-sealing layer laminated on one side of metallic thin film made of aluminum, and a protection film laminated on the other side.
In the film-covered battery using the laminated film as a casing, a battery element generally including a positive electrode, a negative electrode and an electrolyte, is enclosed by the casing so that the thermo-fusing resin film can face inwardly each other, and the casing is thermally fused at the periphery of the battery element to form airtight sealing structure (hereinafter simply referred to as “sealing”). There may be employed, for example, polyethylene or polypropylene film for the thermo-fusing resin film, and nylon or polyethylene-terephthalate film for the protection film. The battery element has positive and negative electrode leads connected with the positive and negative electrodes, respectively, for drawing them outside the casing. When sealing the battery element, these electrode leads are protruded from the casing, and then the casing is thermally fused.
If a voltage over a rated range is applied to a battery during use of the battery, electrolytic solution is possible to be electrolyzed to generate gas, and the rise of pressure inside the battery results. If the battery is used at a high temperature over a rated range, electrolysis of electrolyte base or the like also causes to produce gas generating substance. The use of a battery within the rated range is basically ideal for preventing gas generation. However, if failure of a control circuit for the battery occurs due to some causes, and a resultant abnormal voltage is applied or surrounding temperature abnormally rises, large quantities of gas may be possibly generated.
The gas generation inside the battery causes the pressure inside the battery to be raised. Extreme rise of the inside pressure is possible to cause the battery to be exploded if worst comes to worst. In order to avoid the explosion of the battery, the battery using a metal can as the casing generally has a pressure relief valve for releasing the gas outside the battery when the pressure inside the battery goes up. However, the film-covered battery, which uses the casing films, is difficult to adopt a pressure relief valve because of its structure. When the inside pressure excessively goes up in the film-covered battery, the casing films expand, and finally explodes to eject the gas from a broken position, which is possible to affect surrounding equipment according to the broken position, because it is difficult to determine the position to break.
In order to solve the problem due to gas generation inside the battery, in a conventional film-covered battery, Japanese Laid-Open Patent Publication No. 2000-100399 discloses a film-covered battery in which a battery element is sealed under a condition that a part of thermally fused portion is set to lower temperature than the other part. Japanese Laid-Open Patent Publication No. 11-97070 discloses another film-covered battery in which a part of thermally fused portion has been made with thermally non-fusible resin sheet intervened. In both examples, a part of the casing has been made with lower thermo-fusing strength of films, the part acting as a safety valve.
Further, Japanese Laid-Open Patent Publication No. 2002-56835 discloses a structure in which a part of thermally fused portion is cut off from outside. Japanese Laid-Open Patent Publication No. 10-55792 discloses another structure in which a part of a sealed side has an un-bonded portion that becomes smaller in width outward from the inside of the battery. Each of these structures has on the thermally fused portion a part where the width of the fused portion partially becomes narrower outward from the inside of the battery, the part functioning as a safety valve.
However, the structures for releasing the gas in the conventional film-covered batteries described above have a risk of trouble, such as the leakage of electrolyte, due to the deterioration of the thermally fused portion. Additionally, it has been difficult to adjust the thermo-fusing strength, that is, difficult to accurately set the release pressure at the time of occurrence of abnormality.
In order to realize practical release pressure using the above-described conventional structure which has the thermally fused portion with the width partially narrowed, the narrowed part functioning as a safety valve should have a very narrow fused width of, for example, 1 mm. This structure has caused the sealing reliability to be reduced, and the width error of fused portion at the time of manufacturing has greatly affected the release pressure, resulting in difficulty accurately setting the release pressure. The reason for necessitating such a narrow fused width is that: a film-covered battery tends to have weaker sealing strength at the area where electrode leads extend out for taking out current, and therefore the safety valve has to be opened in advance with substantially lower pressure in order to prevent gas release through the lead extending area.
The problem described above commonly arises not only in the batteries but also in film-covered electric devices in which an electric device element, having the possibility of generating gas, is sealed with casing films.